By way of example, electricity meters are known that have a box containing in particular the consumption measurement unit and the terminal strip for connecting the meter firstly to the public network and secondly to the user's electricity installation.
The portion of the box receiving the terminal strip is covered by a removable cover that enables technicians to access the terminal strip in order to connect the meter to the installation. The cover is held in position on the box by means of a quarter-turn screw held captive in the cover and turning between a locking position and an unlocking position.
If the user of the meter seeks to tamper with consumption measurement, it is necessary to have access to the terminal strip. In order to reveal that such access has occurred, orifices are provided in register with the head of the screw and the cover so as to pass a lead-sealed wire, with breakage of the seal enabling a technician to detect that the movable cover has been removed in non-authorized manner. However, technicians visit quite rarely and such visits are becoming rarer still as a result of the development of remote meter reading.
Suggestions have been made to place a switch in the box that is actuated by the screw being taken into the unlocking position, the switch serving to send a warning to the network in order to reveal tampering without it being necessary to take action on the meter. Nevertheless, that assumes that the tampering takes place while the meter is powered, which means that a fraudulent person could take advantage of a power cut, e.g. while work is being performed, for the purpose of gaining access to the terminal strip without being detected.
In order to remedy that drawback, an electricity meter is known having a box containing an electronic circuit for detecting opening of the cover when the emitter is not powered. The screw that is mounted on the cover to pivot between a locking position and an unlocking position has an end provided with at least one tooth for meshing with the teeth of a code wheel that is mounted to pivot in one direction only over pads of the detection circuit. The position of the code wheel relative to the pads of the detection circuit is stored each time the meter is powered in such a manner that a simple comparison serves to reveal whether or not the screw was moved during an interruption of the power supply to the meter. Nevertheless, it is possible for a fraudulent person to put the code wheel back into position by causing it to revolve through one complete revolution before the power supply is reestablished.